This application claims the priority of German Application No. 198 15 711.8, filed Apr. 8, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for controlling or automatically controlling the engine brake of a motor vehicle, and more particularly, to a method in which, in the coasting operation during a transmission shifting operation, the resulting braking torque acting upon the engine is reduced.
U.S. Pat. No. 5,088,348, shows an engine braking system for a motor vehicle which consists of a closable valve in the exhaust gas system of the internal-combustion engine and an associated valve in the intake system. In order to generate braking power in the coasting operation and brake the vehicle, the valve in the exhaust gas system can be closed by an electrically operable control element so that an excess pressure is built up in the exhaust gas system which counteracts the piston movement.
In the motor vehicle, an automatic transmission is used which automatically triggers a shifting change as soon as the vehicle speed falls below a defined value and the accelerator pedal is simultaneously not acted upon. During the shifting change, the transmission is uncoupled from the engine so that no external output torque can be transmitted to the engine by the transmission, and the engine is acted upon without any opposing output torque only by the braking power of the engine brake. In order to avoid the rotational engine speed of the internal-combustion engine from falling considerably as the result of the braking power and a back kick taking place onto the transmission, a control signal is generated in a control system. The control signal is supplied to the control element of the valve in the exhaust gas system whereupon the engine brake is rendered inoperative during a shifting change. As a result, a falling of the rotational engine speed is prevented by the engine brake so that both blades of the converter of the automatic transmission rotate approximately at the same rotational speed.
This known system has the disadvantage that, because of the inherently dynamic behavior of the components of the engine brake, a precise adjustment of the time period during which the engine brake is rendered inoperative cannot be sufficiently achieved. A premature or late disconnection or reconnection of the engine brake results either in an unnecessary loss of braking power or in an undesirably large drop of the rotational engine speed.